1. Field of the Invention
The present invention relates to a process for production of starch sugars wherein a high-purity glucose product and a high-purity maltose product are produced from a starch. More particularly, the present invention relates to a process for producing starch sugars at high productivity, wherein by the use of a particular chromatographic fractionation technique, the liquefaction and saccharification of starch are simplified; a glucose fraction with a glucose purity of at least 97% by weight ("purity" hereinafter denotes a percentage by weight which a starch sugar such as glucose accounts for of all starch sugars present), a maltose fraction with a maltose purity of at least 80% by weight, and an oligosaccharide fraction can be produced very efficiently in one operation; and all the fractions withdrawn can be made into marketable starch sugar products.
2. Description of the Prior Art
Glucose has conventionally been produced by liquefying starch milk with a liquefaction enzyme typified by .alpha.-amylase, saccharifying the liquefied starch with a saccharification enzyme typified by glucoamylase, subjecting the saccharified starch to purification steps such as filtration, decolorization, desalting and the like, and concentrating the resulting material. In order to produce glucose of high purity by this conventional process, the occurrence of oligosaccharides having polymerization degrees of 2 or more must be minimized during the liquefaction and saccharification steps. This requires strict control of the pH, temperature, salt concentration, reaction time, etc. in the enzymatic reactions and, in some cases, a debranching enzyme is also used in the saccharification step in order to minimize the occurrence of oligosaccharides of high polymerization degrees (the use of the debranching enzyme makes difficult the operation control). The purity of glucose industrially obtained by the above conventional process is considered to be about 97% by weight or less. When glucose is used for medicinal applications or for production of sorbitol (a starting material for vitamin C) requiring glucose having a purity higher than 97% by weight, it is a general practice to further purify, by crystallization, the glucose obtained by the above process, having a purity of about 97% by weight or less, and to use the resulting purer crystalline glucose for said applications.
In order to increase the glucose purity of the aqueous starch sugars solution with a glucose purity of about 97% by weight, obtained by the conventional process for liquefaction and saccharification, a method is known in which the portion other than glucose fraction is separated and removed as an oligosaccharide fraction by the use of a chromatographic separator of simulated moving bed type for fractionation into two fractions [Japanese Patent Application Kokai (Laid-Open) No. 83991/1991]. In this method, however, since the feed for chromatographic fractionation is an aqueous starch sugars solution with a high purity of glucose which is obtained only via the complicated liquefaction and saccharification steps of conventional technique, the simplification of the liquefaction and saccharification steps is impossible; moreover, since the chromatographic fractionation is fractionation into two fractions, it is impossible to separate at any high purity the maltose which is assumed to be present in the oligosaccharide fraction.
On the other hand, maltose has conventionally been produced by liquefying starch milk with a liquefaction enzyme typified by .alpha.-amylase, saccharifying the liquefied milk using, in combination, .beta.-amylase and a debranching enzyme, subjecting the saccharified starch to purification steps such as filtration, decolorization, desalting and the like, and concentrating the resulting material. In order to produce maltose of high purity by this conventional process, the occurrence cf glucose and oligosaccharides having polymerization degrees of 3 or more must be minimized during the liquefaction and saccharification steps. This requires strict control of the pH, temperature, salt concentration, reaction time, etc. in the enzymatic reactions. Thus, while the above conventional process, when applied industrially, gives maltose having a purity of about 70-80% by weight, considerable technical and economic difficulties are encountered in order to obtain, by the process, maltose having a purity of 80% by weight or more.
In order to increase the maltose purity of the aqueous starch sugars solution containing maltose in a purity of about 70% by weight, obtained by the conventional process for liquefaction and saccharification, a process is known in which a maltose fraction is separated by the use of a chromatographic separator of simulated moving bed type for fractionation into two fractions (Japanese Patent Publication No. 51120/1987). In this process, however, since the feed for chromatographic fractionation is an aqueous starch sugars solution which is obtained only via the complicated liquefaction and saccharification steps of conventional technique, the simplification of the liquefaction and saccharification steps is impossible; moreover, since the chromatographic fractionation is fractionation into two fractions, it is impossible to separate each of glucose and oligosaccharides in a high purity.
In order to produce maltose of an increased purity from a solution containing maltose with a purity of about 80-90% by weight and further containing glucose and oligosaccharides, U.S. Patent Nos. 5,198,120 and 5,223,143 assigned to the assignee of the present application state a process for producing high-purity maltose by separating and removing fractions other than maltose fraction in the forms of a glucose-rich fraction and an oligossacharide-rich fraction using a chromatographic separator of simulated moving bed type capable of conducting fractionation into three or more fractions, at an efficiency far higher than that of the conventional chromatographic separator used for fractionation into two fractions (Example 2 of U.S. Patent No. 5,198,120 and Example 2 of U.S. Patent No. 5,223,143). This process enables fractionation into three fractions in one operation, thereby separating maltose at a high purity. However, the glucose fraction and the oligosaccharide fraction each contains other undesirable components, making it impossible to obtain high-purity glucose or high-purity oligosaccharides. Moreover, since the feed for chromatographic fractionation is a multi-component liquid in which maltose is highly concentrated only by the complicated liquefaction and saccharification steps, it is not possible to simplify the manufacturing process by doing without these complicated steps.
In each of the foregoing conventional processes, the product obtained is ordinarily high-purity glucose alone or high-purity maltose alone because the product is obtained by predominantly producing a target sugar with a minor amount of other sugars. Consequently, when glucose is produced, maltose and any other oligosaccharides become impurities and it is necessary to minimize the occurrence of oligosaccharides having molecular weights the same as or larger than maltose molecular weight, by strictly controlling the enzymatic reactions in the liquefaction and saccharification steps. Even by exercising such control, the maximum possible purity of glucose obtained is about 97% by weight. Hence, in order to obtain glucose of higher purity, the above-mentioned chromatographic fractionation or crystallization must have been used additionally.
When maltose is produced, glucose and oligosaccharides having molecular weights the same as or larger than maltotriose molecular weight become impurities and it is necessary to keep low the occurrence of glucose (a monosaccharide) and oligosaccharides having molecular weights the same as or larger than maltotriose molecular weight, by strictly controlling the enzymatic reactions in the liquefaction and saccharification steps. Even by exercising such control, the maximum purity of maltose industrially obtainable is about 80% by weight. Hence, in order to obtain maltose of higher purity, the above-mentioned chromatographic fractionation or crystallization must have been used additionally.
In the conventional chromatograhic separation process, since the fractionation efficiency is not sufficiently high, the intended component recovered is generally one component with the other components being impurities. Further, the feed for chromatographic fractionation is a solution wherein said intended component has been concentrated beforehand by some means so as to make it into a major component. Thus, it has been impossible at least industrially to obtain a plurality of fractions each containing a different component in a high purity, by the conventional chromatographic fractionation.
Of the hitherto proposed chromatographic separators, one considered to be most efficient is a chromatographic separator of simulated moving bed type comprising a plurality of columns which are packed with an adsorbent and which are connected by fluid paths (pipes) so as to form an endless serial circulation channel. In the conventional chromatographic separator of simulated moving bed type, however, separation of components is conducted merely by sequentially shifting the position for feeding the feed, the position for feeding the eluant and the positions for withdrawing eluates. Accordingly, the conventional simulated moving bed type separator is designed to separate a feed into two fractions. When fractionation of a feed into three fractions using such a separator is necessary, the fractionation must be conducted in two stages (two times), which is undesirable from the operational and cost standpoints.
As mentioned previously, there are disclosed a chromatographic separator of simulated moving bed type capable of fractionating a feed into three or more components in one operation and a fractionation process using the separator, in U.S. Patent No. 5,198,120 and U.S. Patent No. 5,223,143 both by the assignee of the present application. The feature of the chromatographic separator lies in that a shutoff valve is provided at a particular position of an endless serial circulation channel and a feed feeding path is connected with the circulation channel at one position just downstream of the shutoff valve, that is, the position for feeding the feed is not shifted and the circulation channel can be shut off as necessary. This chromatographic separator, however, has the following drawbacks. Although this separator is designed to fractionate a feed into three or more components, there is no actual case wherein the fraction each of glucose, maltose and oligosaccharides having higher polymerization degrees than maltose has been separated in a high purity. In Example 2 of each of the U.S. Patent Nos. 5,198,120 and 5,223,143, for instance, a case of separation of high-purity maltose is described as mentioned previously; however, the glucose fraction and the oligosaccharide fraction each contain other components, and neither high-purity glucose nor high-purity oligosaccharides are obtained. Further, since the feed for the chromatographic fractionation is a multi-component solution wherein, the maltose concentration has been controlled at a high level beforehand by complicated liquefaction and saccharification steps, any simplification of the liquefaction and saccharification steps is no longer feasible.